limit tests that are run on CRAN

[ginberg]

Check: Overall checktime, Result: NOTE
Overall checktime 11 min > 10 min

mainly from

• checking tests ... [442s] OK
  Running 'testBigQuery.R'
  Running 'testOracle.R'
  Running 'testPostgres.R'
  Running 'testRedshift.R'
  Running 'testSnowflake.R'
  Running 'testSpark.R'
  Running 'testSqlServer.R'
  Running 'testSqlite.R' [438s]

This is close to the 10 min limit, so I let it pass now (hence on its
way to CRAN), but for the next time:

Please reduce the test timings by using

• small toy data only
• few iterations
• or by running less important tests only conditionally if some
  environment variable is set that you only define on your machine?